For example, a capacitor is provided with two electrodes, that is, an anode and a cathode. As the anode material, valve metals such as aluminum and tantalum on which surface an insulating oxide film can be formed are used. As the cathode material, any of electrolytes, inorganic semiconductors, organic conductive materials and metal thin films may be used. When the cathode material is an electrolyte, an aluminum foil having an enlarged surface area as the cathode terminal is frequently used. This type of aluminum foil is called an aluminum foil for electrolytic capacitor cathode. The surface area of a cathode aluminum foil is enlarged to increase the electrical capacity of a capacitor.
In the meantime, a nonaqueous electrolyte is used in an aluminum electrolytic capacitor having a proof temperature exceeding 105° C. The nonaqueous electrolyte is different from an ethylene glycol-water type electrolytic solution having 10% or more of water and means an electrolyte in which water is not contained at all or is contained in an amount of 5% or less if contained. In an aqueous electrolytic solution, water contained therein is electrodialyzed when voltage is applied to a capacitor with the result that the produced hydroxyl ions are combined with cationic ammonium ions or amidinium ions on the cathode side to generate alkali compounds. For this, the cathode foil is corroded in the capacitor which causes reduced capacity as a change with time. It is therefore demanded of an aqueous electrolyte aluminum electrolytic capacitor to have stability with time as its characteristics. Similarly, it is also demanded of a nonaqueous electrolyte aluminum electrolytic capacitor to have stability with time.
Therefore, the cathode aluminum foil needs to have stability with time besides high electrical capacity as the characteristics to be required.
In order to increase the electrical capacity of the cathode aluminum foil, a method is usually adopted in which the surface area of the aluminum foil is enlarged by etching. However, since the surface of the aluminum foil etched is merely coated with a natural oxide film, there is a problem that the electrical capacity deteriorates during contacting with the electrolyte for a long period of time.
An electrode whose surface is enlarged by allowing a carbon powder to adhere to the surface of an aluminum foil has been developed. In the publication of Japanese Patent Application Laid-Open (JP-A) No. 2000-164466 (Patent Reference 1), a method is disclosed in which a collector of an aluminum foil is provided with an intermediate film of carbon or an intermediate layer of a metal nobler than an aluminum foil and an active material layer such as carbon layer is applied to the intermediate layer.
However, even if the above production method is used, the obtained cathode foil has insufficient adhesion between the carbon powder and the aluminum foil. Therefore, there may be sometimes a phenomenon that the carbon powder is peeled from the surface of the aluminum foil when the capacitor is charged or discharged occurs. As a result, there is the problem that the stability of the capacitor deteriorates with time.
Also, an improvement in the adhesion between the carbon powder as a conductive material and the aluminum foil is an essential technical theme to impart conductivity to not only capacitor electrode materials but also electrode materials constituting battery electrodes such as a positive electrode of a lithium ion battery and electrode collectors. For example, when the carbon powder is adsorbed to the aluminum foil by using a general binder according to conventional technologies, since the internal resistance of the positive electrode increase, there is a problem that the time required to charge a lithium ion battery is longer.
Patent Document 1: Publication of JP-A No. 2000-164466